The most important thing that makes Adsense work is one and only 'The Traffic'. You have to create more traffic to your website to make money from Adsense, at least 10,000 unique visitors a month and from that maybe 3-5% (300-500) will click the ads. The most you can get if you are lucky would be 10%. There are trends in internet market where some of the months you will find that you get the highest clicks and some of the months you get very little. If you have the money, you can purchase the traffic from reliable suppliers, which is not cheap. The cheapest way to increase traffic is through Blogging where you can register your blog with Tagging Sites like Del.icio.us, Technorati, Digg, Spurl, Reddit, Ask, Jots, etc.
To promote your site is not an easy task if you don't want to buy traffic. Your site may get good traffic with highest clicks after 6 months of promotion, provided that you have promoted your sites to all RSS, Feed, Tagging and Ping directories and keep updating it with good and high quality contents.
The second is the placement of the ad. Some say the placement at the top will be more visible, some say it should be placed near your profile in the side bar because visitors always want to know about you and another says it will be more effective to place them inside the contents. As there are three type of ads, the search one will be the less popular. I really think it depends on your blog layout, but the most reliable would be the contextual ad that is placed at the top inside the content not at the title.
So good luck for those who really want to make money from zero investment with Google Adsense. It will work for you because it is already proven works for those who already earned from Adsense.